1. Field of the Invention
This invention relates broadly in the field of electronic generators and in particular is concerned with an improvement for simultaneously producing voices at different pitches in musical tone generators.
2. Description of Prior Art
One of the tonal requirements for an electronic musical instrument is the capability of generating voices, or tones, at a variety of pitches. The pitch of a voice is fundamental frequency. In electronic musical instruments of the organ type, pitches are usually expressed in feet in accordance with pipe organ terminology. Thus an 8-foot pitch is the "normal" pitch because the note A.sub.4 will have at a fundamental frequency of 440 hz. When an 8-foot stop, or tone switch, is actuated tones are produced at the same pitch as the selected key. Thus if the keyboard switch is actuated, a note will be sounded at the pitch having a fundamental frequency of 440 hz. If a 16-foot stop is selected, the instrument will produce tones at a pitch one octave lower than the actuated keyboard switch. That is if the keyboard switch corresponding to A.sub.4 is actuated, the output tone will have a pitch corresponding to A.sub.3. Higher voice pitches are also implemented. For example, a 4-foot stop and a 2-foot stop respectively result in producing notes one and two octaves higher than the actuated keyboard switch.
Frequently the musician will combine different voice pitches to obtain a new composite tone color. If a 16-foot and 8-foot step are used simultaneously, the instrument will produce a composite sound of the unison, or 8-foot pitch, and one an octave lower. Higher pitches can also be added by using stops of shorter than 8-foot pitch. Thus a 1 3/5 foot voice may be used to enhance, or add to, the fifth and tenth harmonics of a simultaneously selected 8-foot stop.
In electronic musical tone generators such as the Polyphonic Tone Synthesizer described in U.S. Pat. No. 4,085,644 and the Computor Organ described in U.S. Pat. No. 3,809,786 musical waveshapes are generated by a computational algorithm. These computational algorithms are of the Fourier type in which the waveshape, or tonal structure, is determined by selected sets of harmonic coefficients. The stop switches are used to select these harmonic coefficients which reside in addressable memories. For such tone generators, all standard organ footages shorter than 8-foot can be generated by supressing selected harmonics as described in detail in U.S. Pat. No. 3,809,786. For example, a 4-foot pitch tone is created by using the basic 8-foot generator and selecting a set of harmonic coefficients having a zero value for all the odd-numbered harmonics. The tonal result is a voice created at an octave above the normal 8-foot generator.
The use of harmonic suppression to generate lower footage voices will restrict the number of harmonics available as a function of the pitch. A 4-foot voice will be limited to one-half of the number of harmonics available for the normal 8-foot tones. A 2-foot voice will have only one-quarter of the number of harmonics available for the normal 8-foot tone.
The simultaneous generation of 16-foot and 8-foot voices in musical systems of the Fourier synthesis type can be accomplished by having the 16-foot pitch as the normal pitch. The 8-foot and shorter footages could be obtained by the method of harmonic suppression. Such systems are impractical in many cases because of the reduction in harmonics for the higher pitches. If the normal or 16-foot pitch is limited to 16 harmonics, then an 8-foot voice having only 8 harmonics is not adequate for the majority of electronic musical instruments.
Prior art systems have resorted to independent tone generators for the 16-foot pitch voices and the 8-foot pitch voices to overcome the problem of reduced harmonics caused by using harmonic suppression to generate the higher voice pitches. A method for simultaneously implementing 16-foot and higher pitched voices for the Computor Organ of U.S. Pat. No. 3,809,786 is described in U.S. Pat. No. 3,809,790 entitled Implementation of Combined Footage In A Computer Organ. The system described in U.S. Pat. No. 3,809,790 contains both an 8-foot tone generator and an independent 16-foot tone generator. In a preferred embodiment 16 time slots are alloted for the generation in real time of a single point on the desired waveshape amplitude. A selected subset of these time slots are allocated to the 8-foot generator and the remaining time slots are allocated to the 16-foot generator. As the preferred embodiment, the 8-foot time spectra is generated with the harmonic sequence 1,2,3,4,5,6, 7,8,10,12,14,16. The sequence of 9,11,13,15 harmonics are not produced by the 8-foot generator. The four missing harmonic time slots for the 8-foot generator are allocated to the 16-foot generator which uses these time slots to generate harmonics 1,3,5,7 of the 16-foot pitch fundamental frequency. The output from the 8-foot and 16-foot tone channels are added together and the sum is then converted to an analog signal to provide the current point on a real-time musical waveform.
Since the 8-foot harmonic components correspond to the even harmonic components of a 16-foot series, the net result is that an effective 16-foot spectrum is generated having the first eight harmonics, every other harmonic up to the 16th, and then every fourth harmonic from the 20th through the 32nd.
The system described in U.S. Pat. No. 3,809,709 for obtaining simultaneous 8-foot and 16-foot tones suffers several disadvantages. First, the system still requires two independent 8-foot and 16-foot tone computation channels. The only saving is in the computation time obtained by time sharing a common set of time slots used for calculation. Second, the output tones having missing harmonics which places a limitation on the possible tone colors that can be created.
The present invention provides a novel implementation for simultaneously generating both 16-foot and 8-foot voices in the basic Polyphonic Tone Synthesizer described in U.S. Pat. No. 4,085,644. Two significant advantages of the instant invention is that the tone creation system does not require separate and independent 8-foot and 16-foot tone generators and the 8-foot tones are created with no loss in the number of harmonics.